Look on craigslist. 33x12.50x15 is a pretty common size, so you should be able to find some. A lot of times they sell them with the rims, and you might be able to either buy them with the rims and then sell the rims later, or try and talk the selled into seperating them. You could get away with your 32x11.50 as a spare, or keep one of your 33x10.50 as a spare.